I also replaced my battery today...
on my 325i. I actually got my battery at NAPA (BT7548). It is about an 1.5 " shorter than the original battery but, it has the vent hole in the right place plus it has 700 CCA compared to my white BMW 640 CCA. Other than that, it was identical to the original in black.
I did check Walmart and Autozone. They did have the batteries mentioned above but, they were shaped a little different and they had the vent hole in the middle.
Just to let everybody know that there is another choice out there...and I only paid around $100. No issues whatsoever with the replacement. I am not planning to register the battery either. If anything, I will get another battery under warranty...